In internal combustion engines, the bearing assemblies typically each comprise a pair of half bearings retaining a crankshaft that is rotatable about an axis. Each half bearing is a hollow generally semi-cylindrical bearing shell, and typically at least one is a flange half bearing, in which the bearing shell is provided with a generally semi-annular thrust washer extending outwardly (radially) at each axial end. In other bearing assemblies it is also known to use an annular or circular thrust washer.
The bearing surfaces of bearing shells generally have a layered construction, in which a substrate comprising a strong backing material is coated with one or more layers having preferred tribological properties to provide a bearing surface that faces a cooperating moving part, e.g. a crankshaft journal, in use. In known bearing shells, a substrate comprises a backing coated with a lining layer, and the substrate is in turn coated with an overlay layer.
The strong backing material may be steel, and may have a thickness of about 1 mm or more. A known lining layer may be a copper-based material (e.g. copper-tin bronze) or an aluminium-based material (e.g. aluminium or aluminium-tin alloy), which is adhered to the substrate (either directly to the backing or to an optional interlayer). The thickness of the lining layer is generally in the range from about 0.05 to 0.5 mm (e.g. 300 μm of copper-based alloy consisting of 8% wt Sn, 1% wt Ni, and balance of Cu, apart from incidentally impurities). The overlay layer may be 6 to 25 μm of a plastic polymer-based composite layer or a metal alloy layer (e.g. a tin-based alloy overlay).
For example, WO2010066396 describes a plastic polymer-based composite material for use as an overlay layer on a copper- or aluminium-based lining layer, which is in turn bonded onto a steel backing. The described overlay layer comprises a matrix of a polyamide-imide plastic polymer material, having distributed throughout the matrix (% vol proportions are specified with respect to the content of the overlay layer after the polymer has been cured): from 5 to less than 15% vol of a metal powder; from 1 to 15% vol of a fluoropolymer particulate, the balance being the polyamide-imide resin apart from incidental impurities (e.g. a layer of 12 μm thickness that comprises 12.5% vol Al, 5.7% vol PTFE particulate, 4.8% vol silane, <0.1% vol other components, and balance (approximately 77% vol) polyamide-imide).
WO2010066396 describes that prior to deposition, the deposition mixture comprises polyamide-imide plastic polymer in a solvent system of N-methyl-2-pyrrolidone (NMP) and xylene, and the solid particulate is suspended in the deposition mixture. Alternatively, it is also known to use a solvent mixture of N-ethyl-2-pyrrolidone (NEP) and xylene. In each case, a small proportion of xylene (e.g. 1 to 2% wt of the deposition mixture) is used to enhance the stability of particulate within the deposition mixture. Such plastic polymer-based overlay layers may be deposited by various different methods, including spraying, pad printing (an indirect offset printing process, e.g. in which a silicone pad transfers a layer of the plastic polymer composite material onto the sliding bearing substrate), screen printing, or by a transfer rolling process. A layer may be deposited as a single coating, or may be built-up as a plurality of separate coatings, with a flash off phase carried out following the deposition of a coating, to remove solvent and enhance the structural integrity of the coating, in particular to prevent the sub-layer from running and to enable the deposition of a further coating, prior to curing.
After the deposition of the overlay layer has been completed, the entire polymer layer is thermally cured by heating to set the polymer-based layer, by inducing cross-linking of the polymer matrix. The overlay layer may be cured at 140 to 280° C. for a duration that may range from a few minutes to a few hours (e.g. 10 minutes to 2 hours). WO2010066396 describes an exemplary final cure of 190° C. for 30 minutes.